In any case, since you are installing Win 7 after XP...removal or deletion of XP afterwards should be no problem. The boot options at startup will stail reflect two options, even though onw is gone...but that is easily corrected.
One note...the C: drive/partition...is not fixed. whichever version of Windows is booted into...automatically becomes the C: partition/drive when looking at all other partitions/drives. So your drive letters may appear to "change" but it's nothing to be concerned about.